Figure 17.45(a) An ultrasonic image is produced by sweeping the ultrasonic beam across the area of interest, in this case the woman’s abdomen. Data are recorded and
analyzed in a computer, providing a two-dimensional image. (b) Ultrasound image of 12-week-old fetus. (credit: Margaret W. Carruthers, Flickr)
How much detail can ultrasound reveal? The image inFigure 17.45is typical of low-cost systems, but that inFigure 17.46shows the remarkable
detail possible with more advanced systems, including 3D imaging. Ultrasound today is commonly used in prenatal care. Such imaging can be used
to see if the fetus is developing at a normal rate, and help in the determination of serious problems early in the pregnancy. Ultrasound is also in wide
use to image the chambers of the heart and the flow of blood within the beating heart, using the Doppler effect (echocardiology).
Whenever a wave is used as a probe, it is very difficult to detect details smaller than its wavelengthλ. Indeed, current technology cannot do quite
this well. Abdominal scans may use a 7-MHz frequency, and the speed of sound in tissue is about 1540 m/s—so the wavelength limit to detail would
beλ=
vw
f
= 1540 m/s
7 ×10^6 Hz
= 0.22 mm. In practice, 1-mm detail is attainable, which is sufficient for many purposes. Higher-frequency ultrasound
would allow greater detail, but it does not penetrate as well as lower frequencies do. The accepted rule of thumb is that you can effectively scan to a
depth of about 500 λinto tissue. For 7 MHz, this penetration limit is500×0.22 mm, which is 0.11 m. Higher frequencies may be employed in
smaller organs, such as the eye, but are not practical for looking deep into the body.
Figure 17.46A 3D ultrasound image of a fetus. As well as for the detection of any abnormalities, such scans have also been shown to be useful for strengthening the
emotional bonding between parents and their unborn child. (credit: Jennie Cu, Wikimedia Commons)
In addition to shape information, ultrasonic scans can produce density information superior to that found in X-rays, because the intensity of a reflected
sound is related to changes in density. Sound is most strongly reflected at places where density changes are greatest.
Another major use of ultrasound in medical diagnostics is to detect motion and determine velocity through the Doppler shift of an echo, known as
Doppler-shifted ultrasound. This technique is used to monitor fetal heartbeat, measure blood velocity, and detect occlusions in blood vessels, for
example. (SeeFigure 17.47.) The magnitude of the Doppler shift in an echo is directly proportional to the velocity of whatever reflects the sound.
CHAPTER 17 | PHYSICS OF HEARING 619